The present invention relates to a waveguide structure for use with a pump laser that is designed to counteract pump depletion. The waveguide structure may comprise an erbium-doped waveguide amplifier, a nonlinear waveguide wavelength converter device, or an optical fiber, and is particularly useful in communications systems and wavelength conversion applications.
Optical communications systems are useful for transmitting optical signals over long distances at high speeds. An optical signal, which comprises a series of light pulses, is transmitted from a light source, e.g., a laser, to a waveguide and ultimately to a detector. Optical communications systems typically include a variety of devices (e.g., light sources, photodetectors, switches, optical fibers, amplifiers, filters, and so forth). Amplifiers and filters may be used to propagate the light pulses along the length of the waveguide from the light source to the detector. Recently, demand has been increasing for the efficient, large-scale manufacturing of hybrid integrated opto-electronic devices. There is particular interest in the integration of multiple components onto a single chip and hybridization of various components into a single package.
Waveguide structures and optical fibers transmit light signals with use of internal reflection. Basically, an optical waveguide structure comprises an inner or core region fabricated from a material having a certain index of refraction, and an outer region contiguous the core comprised of a material having a lower index of refraction than the core. As long as the refractive index of the core exceeds that of the outer region, a light beam propagated along the core will be guided along the length of the waveguide. Planar waveguides are flat structures that guide light in essentially the same way as optical fibers. Essentially a planar waveguide structure comprises a strip of material embedded in a substrate where the waveguide strip has a relatively high index of refraction relative to the substrate. Thus, light is guided along the high-index waveguide strip, although the strip may not be entirely surrounded by a lower refractive index material, as the waveguide strip may be exposed to a top layer of air. One or more amplifying systems may be disposed along the waveguide transmission path for amplifying the transmitted signal.
An inefficiency inherent in the operation of opto-electronic devices pertains to depletion of pump light used in devices such as waveguide amplifiers and other optically pumped waveguides such as parametric wavelength converters. A waveguide that operates as an amplifier or a nonlinear wavelength conversion device requires the presence of a pump laser. The pump laser enables the process of amplifying or converting the optical signals. For example, FIG. 1 schematically shows an amplifier utilizing an optical fiber 11. A weak optical signal Is enters from the left, passes through an optical isolator 12 and a filter 13 that blocks pump light and transmits the signal wavelength. The light enters a doped fiber 11, typically a rare-earth doped fiber. The amplifier is then illuminated with light from the pump laser 15 which introduces a pump signal Ps to the fiber to excite the dopant atoms, raise them to a higher energy level, and amplify the weak input signal Is to amplified signal As transmitted through the waveguide. A coupler 16 at the end of the amplifier routes the amplified signal to the output fiber, separating it from the pump light, to produce output signal Os. Notably, although in FIG. 1 the pump laser 15 is shown at the distal end of the waveguide amplifier adjacent the output, light can be pumped into the amplifier adjacent the input, following the direction of the amplified signal As.
In any case, once the pump light is launched into the waveguide, the pump laser field gets absorbed as a function of propagation distance in a linear or nonlinear fashion. This absorption produces pump-depletion as a function of propagation distance. The depletion in turn implies a lower efficiency for the amplification or conversion process due to a reduction in pump power density in the waveguide at distances from the pump laser.
As may be appreciated, those involved in the field of communications systems and electro-optical devices continue to seek to develop new designs to improve device efficiency and performance. In particular, it would be advantageous to have a waveguide structure that results in near constant or less of a reduction in pump power density along the waveguide. These and further advantages are provided by this invention which may appear more fully upon considering the description given below.
Summarily described, the invention embraces an article including a waveguide structure for use with a pump light source, e.g., a pump laser, that preserves pump power density. The waveguide structure comprises a waveguide region having a first index of refraction, and a second region having a second index of refraction contiguous the waveguide region, wherein the second index of refraction is lower than the first index of refraction so that light launched into the waveguide structure is guided within the waveguide region. Additionally, the waveguide region has a pump propagation length which is defined as a select distance between a first pump site and a second pump site and over which pump light is transmitted within the waveguide region for signal amplification or conversion. The effective area of the waveguide region is substantially larger at the first pump site than at the second pump site and thus, the waveguide region is tapered over the pump propagation length, thereby reducing the depletion of pump power density. The waveguide structure may comprise a planar waveguide or an optical fiber.